1. Field of the Invention
This invention generally relates to a surface acoustic wave device having a surface acoustic wave resonator, and more particularly, to a configuration of the surface acoustic wave (hereinafter referred to as SAW) device having a pair of reflection electrodes provided on both sides of a pair of comb-like electrodes.
2. Description of the Related Art
In recent years, wireless devices such as mobile telephones have increasingly been downsized, more advanced and more sophisticated. A high frequency circuit of the wireless device includes a filter of the SAW resonator. The filter is a ladder-type filter or a multimode type filter. The ladder-type filter has a structure in which multiple surface acoustic wave filters are connected in a ladder structure. The multimode type filter includes, for example, a double mode SAW (DMS) filter.
FIG. 1 shows a normal single-port SAW resonator described in Japanese Patent Application Publication No. 6-338756 (hereinafter referred to as Document 1) and Japanese Patent Application Publication No. 10-215145 (hereinafter referred to as Document 2). The SAW resonator includes a pair of comb-like electrodes 10 provided on a piezoelectric substrate. The comb-like electrodes 10, which are alternately interleaved on the piezoelectric substrate, are known as interdigital transducer (IDT). Each of the comb-like electrodes includes a bus bar and electrode fingers that extend from the bus bar in the same direction. A wavelength of the IDT 10 is decided by pitch (period) λIDT between the electrode fingers that extend from the same bus bar. The IDT 10 has a single pitch IDT. A pair of reflection electrodes 12 and 14 are provided on both sides of the IDT 10. The reflection electrodes 12 and 14 shown in FIG. 1 are referred to as a grating-type electrode, and the electrode fingers extending from one bus bar are connected to the other bus bar. Pitches λa and λb of the reflection electrodes 12 and 14 correspond to the periods between the alternate electrode fingers. The reflection electrode 12 has the pitch λa and the reflection electrode 14 has the pitch λb. Here, λa is equal to λb.
The conventional SAW resonators described in Documents 1 and 2; however, have a problem in that resonance characteristics include a ripple component. FIG. 2 is a graph illustrating the resonance characteristics of the SAW resonator shown in FIG. 1. Referring to FIG. 2, the horizontal axis denotes frequency (MHz), and the vertical axis denotes attenuation (dB). As shown in FIG. 2, the curve showing the resonance characteristics includes multiple peaks, namely, the ripple component. FIG. 3 shows filter characteristics in the case where the filter is composed of the above-mentioned multiple SAW resonators. Referring to FIG. 3, multiple ripple components are included in a passband. In this manner, it is hard to form a flat passband with the conventional filter.